Metrology method, patterning device, apparatus and computer program

ABSTRACT

A method of measuring overlay uses a plurality of asymmetry measurements from locations (LOI) on a pair of sub-targets (1032, 1034) formed on a substrate (W). For each sub-target, the plurality of asymmetry measurements are fitted to at least one expected relationship (1502, 1504) between asymmetry and overlay, based on a known bias variation deigned into the sub-targets. Continuous bias variation in one example is provided by varying the pitch of top and bottom gratings (P1/P2). Bias variations between the sub-targets of the pair are equal and opposite (P2/P1). Overlay (OV) is calculated based on a relative shifht (xs) between the fitted relationships for the two sub-targets. The step of fitting asymmetry measurements to at least one expected relationship includes wholly or partially discounting measurements (1506, 1508, 1510) that deviate from the expected relationship and/or fall outside a particular segment of the fitted relationship.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 16/594,613, filed on Oct. 7, 2019, which claimspriority to European Patent Application 18199182.9, filed on Oct. 8,2018, the entire contents of all of which are incorporated herein byreference.

BACKGROUND Field of the Invention

The present invention relates to methods and apparatus for metrologyusable, for example, in the manufacture of devices by lithographictechniques and to methods of manufacturing devices using lithographictechniques. The invention further relates to patterning devices andcomputer program products usable in such methods.

Background Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. In lithographic processes, itis desirable frequently to make measurements of the structures created,e.g., for process control and verification. Various tools for makingsuch measurements are known, including scanning electron microscopes,which are often used to measure critical dimension (CD), and specializedtools to measure overlay, a measure of the accuracy of alignment of twolayers in a device. Overlay may be described in terms of the degree ofmisalignment between the two layers, for example reference to a measuredoverlay of 1 nm may describe a situation where two layers are misalignedby 1 nm.

Recently, various forms of scatterometers have been developed for use inthe lithographic field. These devices direct a beam of radiation onto atarget and measure one or more properties of the scatteredradiation—e.g., intensity at a single angle of reflection as a functionof wavelength; intensity at one or more wavelengths as a function ofreflected angle; or polarization as a function of reflected angle—toobtain a “spectrum” from which a property of interest of the target canbe determined. Determination of the property of interest may beperformed by various techniques: e.g., reconstruction of the target byiterative approaches such as rigorous coupled wave analysis or finiteelement methods; library searches; and principal component analysis.

The targets used by conventional scatterometers are relatively large,e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spotthat is smaller than the grating (i.e., the grating is underfilled).This simplifies mathematical reconstruction of the target as it can beregarded as infinite. However, in order to reduce the size of thetargets, e.g., to 10 μm by 10 μm or less, e.g., so they can bepositioned in amongst product features, rather than in the scribe lane,metrology has been proposed in which the grating is made smaller thanthe measurement spot (i.e., the grating is overfilled). Typically suchtargets are measured using dark field scatterometry in which the zerothorder of diffraction (corresponding to a specular reflection) isblocked, and only higher orders processed. Examples of dark fieldmetrology can be found in international patent applications WO2009/078708 and WO 2009/106279 which documents are hereby incorporatedby reference in their entirety. Further developments of the techniquehave been described in patent publications US20110027704A,US20110043791A and US20120242970A. Modifications of the apparatus toimprove throughput are described in US2010201963A1 and US2011102753A1.The contents of all these applications are also incorporated herein byreference. Diffraction-based overlay using dark-field detection of thediffraction orders enables overlay measurements on smaller targets.These targets can be smaller than the illumination spot and may besurrounded by product structures on a wafer. Targets can comprisemultiple gratings which can be measured in one image.

In the known metrology technique, overlay measurement results areobtained by measuring an overlay target twice under certain conditions,while either rotating the overlay target or changing the illuminationmode or imaging mode to obtain separately the −1st and the +1stdiffraction order intensities. The intensity asymmetry, a comparison ofthese diffraction order intensities, for a given overlay target providesa measurement of asymmetry in the target. This asymmetry in the overlaytarget can be used as an indicator of overlay (undesired misalignment oftwo layers).

In the known method using four distinct sub-targets, a certain portionof the patterned area is not usable due to edge effects. Insemiconductor product designs the efficient use of space is veryimportant. The use of only two specific offsets enforces the aboveassumption of linearity, which may lead to inaccuracy when the truerelationship is non-linear. To increase the number of offsets in theknown designs used would increase the space used.

SUMMARY OF THE INVENTION

It would be desirable to be able to perform metrology of overlay orother performance parameters with increased accuracy, and/or with lessspace used for the targets.

The invention in a first aspect provides a method of measuring aperformance parameter of a lithographic process, as defined in appendedclaim 1.

The invention in a second aspect further provides a patterning devicefor use in a lithographic apparatus, the patterning device comprisingportions that define one or more device patterns and portions thatdefine one or more metrology patterns, the metrology patterns includingat least one target for use in a method of the first aspect of theinvention as set forth above, the target having a bias variation betweenlocations on the target, said bias variation being in anasymmetry-related property.

The invention in a further provides a metrology apparatus comprising: anillumination system configured to illuminate with radiation a target; adetection system configured to detect scattered radiation arising fromillumination of the target; wherein said metrology apparatus is operableto perform the method of the first aspect of the invention as set forthabove.

The invention further provides a computer program comprising processorreadable instructions which, when run on suitable processor controlledapparatus, cause the processor controlled apparatus to perform themethod of the first aspect, and a computer program carrier comprisingsuch a computer program.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a lithographic cell or cluster according to an embodimentof the invention;

FIGS. 3(a)-3(d) comprise 3(a) a schematic diagram of a dark fieldscatterometer for use in measuring targets using a first pair ofillumination apertures, 3(b) a detail of diffraction spectrum of atarget grating for a given direction of illumination 3(c) a second pairof illumination apertures providing further illumination modes in usingthe scatterometer for diffraction based overlay measurements and 3(d) athird pair of illumination apertures combining the first and second pairof apertures;

FIG. 4 depicts a known form of multiple grating target and an outline ofa measurement spot on a substrate;

FIG. 5 depicts an image of the target of FIG. 4 obtained in thescatterometer of FIG. 3;

FIG. 6 depicts a first example of a multiple grating target includingcontinuous bias features according to an aspect of the presentdisclosure;

FIG. 7 depicts an image of the target of FIG. 6 obtained in thescatterometer of FIG. 3;

FIGS. 8(a)-8(b) show in schematic detail the implementation ofcontinuous bias in one grating of the target of FIG. 6 under conditionsof 8(a) zero overlay and 8(b) non-zero overlay, according to oneembodiment of the present disclosure;

FIG. 9 shows in schematic detail the arrangement of continuous biasgratings in the multiple grating target of FIG. 6, according to oneembodiment of the present disclosure;

FIG. 10 depicts a second example of a modified multiple grating targetincluding continuous bias features according to an aspect of the presentdisclosure;

FIG. 11 depicts an image of the target of FIG. 10 obtained in thescatterometer of FIG. 3;

FIGS. 12(a)-12(b) show 12(a) in schematic detail the implementation ofcontinuous bias in one grating of the target of FIG. 10, while 12(b)shows the variation of bias with position in such a grating;

FIGS. 13(a)-13(b) show in schematic detail the implementation ofcontinuous bias in two gratings of the multi-grating target of FIG. 10under conditions of 13(a) zero overlay and 13(b) non-zero overlay,according to one embodiment of the present disclosure;

FIG. 14 is a flowchart showing the steps of an overlay measurementmethod using the scatterometer of FIG. 3;

FIGS. 15(a)-15(b) illustrate 15(a) signal processing in relation to thefirst one of the gratings shown in FIGS. 13 and 15(b) signal processingin relation to the other of the gratings shown in FIG. 13, includinggraphical illustration of the principles of calculation of overlay errorin one embodiment of the present disclosure;

FIG. 16 shows in schematic detail implementation of continuous bias intwo gratings of a modified multi-grating target, including the provisionof anchor points according to another embodiment of the presentdisclosure;

FIGS. 17(a)-17(c) illustrate 17(a) the inclusion of bias slope changesas an example of anchor points in the multi-grating target of FIG. 16,17(b) asymmetry signals obtained from the gratings shown In FIG. 16under conditions of non-zero overlay, and 17(c) correction of theasymmetry signals using knowledge of the anchor points;

FIG. 18 illustrates an example of a grating having multi-step bias as analternative to continuous bias, according to another example of thepresent disclosure;

FIG. 19 illustrates a multi-grating target having dual bias gratingsaccording to another example of the present disclosure;

FIG. 20 illustrates an alternative grating target having overlay bias intwo directions, based on L-shaped features;

FIG. 21 illustrates a modified version of the grating of FIG. 20,modified to include a multi-step arrangement of bias regions;

FIG. 22 illustrates another modified version of the grating of FIG. 20,modified to include continuous bias by rotation of L-shaped features,according to yet another embodiment of the present disclosure; and

FIGS. 23 and 24 illustrate targets having multi-step arrangement of biasregions in four quadrants, based on the arrangement of FIG. 21.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Before describing embodiments of the invention in detail, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination optical system (illuminator) IL configured tocondition a radiation beam B (e.g., UV radiation or DUV radiation), apatterning device support or support structure (e.g., a mask table) MTconstructed to support a patterning device (e.g., a mask) MA andconnected to a first positioner PM configured to accurately position thepatterning device in accordance with certain parameters; a substratetable (e.g., a wafer table) WT constructed to hold a substrate (e.g., aresist coated wafer) W and connected to a second positioner PWconfigured to accurately position the substrate in accordance withcertain parameters; and a projection optical system (e.g., a refractiveprojection lens system) PS configured to project a pattern imparted tothe radiation beam B by patterning device MA onto a target portion C(e.g., including one or more dies) of the substrate W.

The illumination optical system may include various types of optical ornon-optical components, such as refractive, reflective, magnetic,electromagnetic, electrostatic or other types of components, or anycombination thereof, for directing, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support may be a frame or a table, for example, whichmay be fixed or movable as required. The patterning device support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support (e.g., mask tableMT), and is patterned by the patterning device. Having traversed thepatterning device (e.g., mask) MA, the radiation beam B passes throughthe projection optical system PS, which focuses the beam onto a targetportion C of the substrate W, thereby projecting an image of the patternon the target portion C. With the aid of the second positioner PW andposition sensor IF (e.g., an interferometric device, linear encoder, 2-Dencoder or capacitive sensor), the substrate table WT can be movedaccurately, e.g., so as to position different target portions C in thepath of the radiation beam B. Similarly, the first positioner PM andanother position sensor (which is not explicitly depicted in FIG. 1) canbe used to accurately position the patterning device (e.g., mask) MAwith respect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan.

Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g., mask) MA, the mask alignment marks may be located betweenthe dies. Small alignment markers may also be included within dies, inamongst the device features, in which case it is desirable that themarkers be as small as possible and not require any different imaging orprocess conditions than adjacent features. The alignment system, whichdetects the alignment markers is described further below.

Lithographic apparatus LA in this example is of a so-called dual stagetype which has two substrate tables WTa, WTb and two stations—anexposure station and a measurement station—between which the substratetables can be exchanged. While one substrate on one substrate table isbeing exposed at the exposure station, another substrate can be loadedonto the other substrate table at the measurement station and variouspreparatory steps carried out. The preparatory steps may include mappingthe surface control of the substrate using a level sensor LS andmeasuring the position of alignment markers on the substrate using analignment sensor AS. This enables a substantial increase in thethroughput of the apparatus.

The depicted apparatus can be used in a variety of modes, including forexample a step mode or a scan mode. The construction and operation oflithographic apparatus is well known to those skilled in the art andneed not be described further for an understanding of the presentinvention.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic system, referred to as a lithographic cell LC or alithocell or cluster. The lithographic cell LC may also includeapparatus to perform pre- and post-exposure processes on a substrate.Conventionally these include spin coaters SC to deposit resist layers,developers DE to develop exposed resist, chill plates CH and bake platesBK. A substrate handler, or robot, RO picks up substrates frominput/output ports I/O1, 1/O2, moves them between the different processapparatus and delivers then to the loading bay LB of the lithographicapparatus. These devices, which are often collectively referred to asthe track, are under the control of a track control unit TCU which isitself controlled by the supervisory control system SCS, which alsocontrols the lithographic apparatus via lithography control unit LACU.Thus, the different apparatus can be operated to maximize throughput andprocessing efficiency.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. Accordingly a manufacturing facility in which lithocell LC islocated also includes metrology system MET which receives some or all ofthe substrates W that have been processed in the lithocell. Metrologyresults are provided directly or indirectly to the supervisory controlsystem SCS. If errors are detected, adjustments may be made to exposuresof subsequent substrates, especially if the inspection can be done soonand fast enough that other substrates of the same batch are still to beexposed. Also, already exposed substrates may be stripped and reworkedto improve yield, or discarded, thereby avoiding performing furtherprocessing on substrates that are known to be faulty. In a case whereonly some target portions of a substrate are faulty, further exposurescan be performed only on those target portions which are good.

Within metrology system MET, an inspection apparatus is used todetermine the properties of the substrates, and in particular, how theproperties of different substrates or different layers of the samesubstrate vary from layer to layer. The inspection apparatus may beintegrated into the lithographic apparatus LA or the lithocell LC or maybe a stand-alone device. To enable most rapid measurements, it isdesirable that the inspection apparatus measure properties in theexposed resist layer immediately after the exposure. However, the latentimage in the resist has a very low contrast—there is only a very smalldifference in refractive index between the parts of the resist whichhave been exposed to radiation and those which have not—and not allinspection apparatuses have sufficient sensitivity to make usefulmeasurements of the latent image. Therefore measurements may be takenafter the post-exposure bake step (PEB) which is customarily the firststep carried out on exposed substrates and increases the contrastbetween exposed and unexposed parts of the resist. At this stage, theimage in the resist may be referred to as semi-latent. It is alsopossible to make measurements of the developed resist image—at whichpoint either the exposed or unexposed parts of the resist have beenremoved—or after a pattern transfer step such as etching. The latterpossibility limits the possibilities for rework of faulty substrates butmay still provide useful information.

A metrology apparatus is shown in FIG. 3(a). A target T and diffractedrays of measurement radiation used to illuminate the target areillustrated in more detail in FIG. 3(b). The metrology apparatusillustrated is of a type known as a dark field metrology apparatus. Themetrology apparatus depicted here is purely exemplary, to provide anexplanation of dark field metrology. The metrology apparatus may be astand-alone device or incorporated in either the lithographic apparatusLA, e.g., at the measurement station, or the lithographic cell LC. Anoptical axis, which has several branches throughout the apparatus, isrepresented by a dotted line O. In this apparatus, light emitted bysource 11 (e.g., a xenon lamp) is directed onto substrate W via a beamsplitter 15 by an optical system comprising lenses 12, 14 and objectivelens 16. These lenses are arranged in a double sequence of a 4Farrangement. A different lens arrangement can be used, provided that itstill provides a substrate image onto a detector, and simultaneouslyallows for access of an intermediate pupil-plane for spatial-frequencyfiltering. Therefore, the angular range at which the radiation isincident on the substrate can be selected by defining a spatialintensity distribution in a plane that presents the spatial spectrum ofthe substrate plane, here referred to as a (conjugate) pupil plane. Inparticular, this can be done by inserting an aperture plate 13 ofsuitable form between lenses 12 and 14, in a plane which is aback-projected image of the objective lens pupil plane. In the exampleillustrated, aperture plate 13 has different forms, labeled 13N and 13S,allowing different illumination modes to be selected. The illuminationsystem in the present examples forms an off-axis illumination mode. Inthe first illumination mode, aperture plate 13N provides off-axis from adirection designated, for the sake of description only, as ‘north’. In asecond illumination mode, aperture plate 13S is used to provide similarillumination, but from an opposite direction, labeled ‘south’. Othermodes of illumination are possible by using different apertures. Therest of the pupil plane is desirably dark as any unnecessary lightoutside the desired illumination mode will interfere with the desiredmeasurement signals.

As shown in FIG. 3(b), target T is placed with substrate W normal to theoptical axis O of objective lens 16. The substrate W may be supported bya support (not shown). A ray of measurement radiation I impinging ontarget T from an angle off the axis O gives rise to a zeroth order ray(solid line 0) and two first order rays (dot-chain line+1 and doubledot-chain line −1). It should be remembered that with an overfilledsmall target, these rays are just one of many parallel rays covering thearea of the substrate including metrology target T and other features.Since the aperture in plate 13 has a finite width (necessary to admit auseful quantity of light, the incident rays I will in fact occupy arange of angles, and the diffracted rays 0 and +1/−1 will be spread outsomewhat. According to the point spread function of a small target, eachorder +1 and −1 will be further spread over a range of angles, not asingle ideal ray as shown. Note that the grating pitches of the targetsand the illumination angles can be designed or adjusted so that thefirst order rays entering the objective lens are closely aligned withthe central optical axis. The rays illustrated in FIGS. 3(a) and 3(b)are shown somewhat off axis, purely to enable them to be more easilydistinguished in the diagram.

At least the 0 and +1 orders diffracted by the target T on substrate Ware collected by objective lens 16 and directed back through beamsplitter 15. Returning to FIG. 3(a), both the first and secondillumination modes are illustrated, by designating diametricallyopposite apertures labeled as north (N) and south (S). When the incidentray I of measurement radiation is from the north side of the opticalaxis, that is when the first illumination mode is applied using apertureplate 13N, the +1 diffracted rays, which are labeled +1(N), enter theobjective lens 16. In contrast, when the second illumination mode isapplied using aperture plate 13S the −1 diffracted rays (labeled −1(S))are the ones which enter the lens 16.

A second beam splitter 17 divides the diffracted beams into twomeasurement branches. In a first measurement branch, optical system 18forms a diffraction spectrum (pupil plane image) of the target on firstsensor 19 (e.g. a CCD or CMOS sensor) using the zeroth and first orderdiffractive beams. Each diffraction order hits a different point on thesensor, so that image processing can compare and contrast orders. Thepupil plane image captured by sensor 19 can be used for focusing themetrology apparatus and/or normalizing intensity measurements of thefirst order beam. The pupil plane image can also be used for manymeasurement purposes such as reconstruction.

In the second measurement branch, optical system 20, 22 forms an imageof the target T on sensor 23 (e.g. a CCD or CMOS sensor). In the secondmeasurement branch, an aperture stop 21 is provided in a plane that isconjugate to the pupil-plane. Aperture stop 21 functions to block thezeroth order diffracted beam so that the image of the target formed onsensor 23 is formed only from the −1 or +1 first order beam. The imagescaptured by sensors 19 and 23 are output to processor PU which processesthe image, the function of which will depend on the particular type ofmeasurements being performed. Note that the term ‘image’ is used here ina broad sense. An image of the grating lines as such will not be formed,if only one of the −1 and +1 orders is present.

The particular forms of aperture plate 13 and field stop 21 shown inFIG. 3 are purely examples. In another embodiment of the invention,on-axis illumination of the targets is used and an aperture stop with anoff-axis aperture is used to pass substantially only one first order ofdiffracted light to the sensor. In other examples, a two quadrantaperture may be used. This may enable simultaneous detection of plus andminus orders, as described in US2010201963A1, mentioned above.Embodiments with optical wedges (segmented prisms or other suitableelements) in the detection branch can be used to separate the orders forimaging spatially in a single image, as described in US2011102753A1,mentioned above. In yet other embodiments, 2nd, 3rd and higher orderbeams (not shown in FIG. 3) can be used in measurements, instead of orin addition to the first order beams. In yet other embodiments, asegmented prism can be used in place of aperture stop 21, enabling both+1 and −1 orders to be captured simultaneously at spatially separatelocations on image sensor 23.

In order to make the measurement radiation adaptable to these differenttypes of measurement, the aperture plate 13 may comprise a number ofaperture patterns formed around a disc, which rotates to bring a desiredpattern into place. Note that aperture plate 13N or 13S can only be usedto measure gratings oriented in one direction (X or Y depending on theset-up). For measurement of an orthogonal grating, rotation of thetarget through 90° and 270° might be implemented. Different apertureplates are shown in FIGS. 3(c) and (d). The use of these, and numerousother variations and applications of the apparatus are described inprior published applications, mentioned above.

FIG. 4 depicts an overlay target or composite overlay target formed on asubstrate according to known practice. The overlay target in thisexample comprises four sub-targets (e.g., gratings) 32 to 35 positionedclosely together so that they will all be within a measurement spot 31formed by the metrology radiation illumination beam of the metrologyapparatus. The four sub-overlay targets thus are all simultaneouslyilluminated and simultaneously imaged on sensor 23. In an examplededicated to measurement of overlay, sub-targets 32 to 35 are themselvescomposite structures formed by overlying gratings that are patterned indifferent layers of the semi-conductor device formed on substrate W.Sub-targets 32 to 35 may have differently biased overlay offsets inorder to facilitate measurement of overlay between the layers in whichthe different parts of the composite sub-targets are formed. The meaningof overlay bias will be explained below with reference to FIG. 7.Sub-targets 32 to 35 may also differ in their orientation, as shown, soas to diffract incoming radiation in X and Y directions. In one example,sub-targets 32 and 34 are X-direction sub-targets with biases of the +d,−d, respectively. Sub-targets 33 and 35 are Y-direction sub-targets withoffsets +d and −d respectively. Separate images of these sub-targets canbe identified in the image captured by sensor 23. This is only oneexample of an overlay target. An overlay target may comprise more orfewer than 4 sub-targets.

FIG. 5 shows an example of an image that may be formed on and detectedby the sensor 23, using the overlay target of FIG. 4 in the apparatus ofFIG. 3, using the aperture plates 13NW or 13SE from FIG. 3(d). While thepupil plane image sensor 19 cannot resolve the different individualsub-targets 32 to 35, the image sensor 23 can do so. The hatched area 40represents the field of the image on the sensor, within which theilluminated spot 31 on the substrate is imaged into a correspondingcircular area 41. Within this, rectangular areas 42-45 represent theimages of the small overlay target sub-targets 32 to 35. If the overlaytargets are located in product areas, product features may also bevisible in the periphery of this image field. Image processor andcontroller PU processes these images using pattern recognition toidentify the separate images 42 to 45 of sub-targets 32 to 35. In thisway, the images do not have to be aligned very precisely at a specificlocation within the sensor frame, which greatly improves throughput ofthe measuring apparatus as a whole.

Once the separate images of the overlay targets have been identified,the intensities of those individual images can be measured, e.g., byaveraging or summing selected pixel intensity values within theidentified areas. Intensities and/or other properties of the images canbe compared with one another. These results can be combined to measuredifferent parameters of the lithographic process. Overlay performance isan important example of such a parameter.

Using for example the method described in applications such asUS20110027704A, mentioned above, overlay error (i.e., undesired andunintentional overlay misalignment) between the two layers within thesub-targets 32 to 35 is measured. Such a method may be referred to asmicro diffraction based overlay (μDBO). This measurement is done throughoverlay target asymmetry, as revealed by comparing their intensities inthe +1 order and −1 order dark field images (the intensities of othercorresponding higher orders can be compared, e.g. +2 and −2 orders) toobtain a measure of the intensity asymmetry.

In a known method using a multi-grating target such as that illustratedin FIG. 4, with overlay OV can be determined via the following equation:

$\begin{matrix}{{OV} = {{\frac{p}{2\pi}{\tan^{- 1}\left( {{\tan\left( \frac{2\pi d}{p} \right)}\left( \frac{\left( {I_{+ d}^{+ 1} - I_{+ d}^{- 1}} \right) + \left( {I_{- d}^{+ 1} - I_{- d}^{- 1}} \right)}{\left( {I_{+ d}^{+ 1} - I_{+ d}^{- 1}} \right) - \left( {I_{- d}^{+ 1} - I_{- d}^{- 1}} \right)} \right)} \right)}} \cong {d\frac{A_{+ d} + A_{- d}}{A_{+ d} - A_{- d}}}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where:

-   -   I_(+d) ⁺¹ is the +1^(st) diffraction order from positive bias        target (e.g., intensity value);    -   I_(+d) ⁻¹ is the −1^(st) diffraction order from positive bias        target;    -   I_(−d) ⁺¹ is the +1^(st) diffraction order from negative bias        target;    -   I_(−d) ⁻¹ is the −1^(st) diffraction order from negative bias        target;    -   A_(−d)=I_(−d) ⁺¹-I_(−d) ⁻¹, (e.g., asymmetry in the +1′ and −1′        intensities from positive bias target); and    -   A_(−d)=I_(−d) ⁺¹-I_(−d) ⁻¹, (e.g., asymmetry in the +1′ and        −1^(st) intensities from negative bias target).

Equation 1 can be reformulated in terms of a sensitivity coefficient Kwhich is a stack dependent parameter having the special property ofbeing overlay independent (assuming a perfect target):

$\begin{matrix}{{{A_{+ d} + A_{- d}} = {K \cdot {OV}}}{{where}\text{:}}} & \left( {{Equation}\mspace{14mu} 2} \right) \\{K = \frac{A_{+ d} - A_{- d}}{d}} & \left( {{Equation}\mspace{11mu} 3} \right)\end{matrix}$

While Equation 2 is a simple linear equation, based on an assumption ofsmall bias values and overlay errors, compared with a pitch of thegratings that form the sub-targets, the dependence of asymmetry onoverlay error and bias over a wider range, has a substantiallysinusoidal form. A sinusoidal model can also be used, instead of thelinear model of Equation 2.

The known method using four distinct sub-targets requires borders aroundeach sub-target (not shown in FIGS. 4 and 5) to make them distinctive inthe image 40. This means that a certain portion of the patterned area isnot usable due to edge effects. Additionally, the use of only twospecific offsets enforces the above assumption of linearity, which maylead to inaccuracy when the true relationship is non-linear.

In the following, we disclose solutions including overlay targets withcontinuous variation of bias, and/or multiple bias values. When appliedin the image plane overlay measurement techniques just described, themultiple bias values can be seen in an intensity image over the targetarea. Verification of linearity and/or sinusoidal fitting can beperformed to ensure that quality information is being used.Additionally, more information about the sensitivity of the target andthe measurement apparatus to overlay and other factors can be obtained.Embodiments will be illustrated based on rotation or staggering of oneor both gratings forming an overlay grating. Embodiments will beillustrated based on different pitches of top and bottom gratings. Withappropriate design, more of the current area can be used in the signaldetermination. Target size may be reduced, and/or measurement accuracyincreased, compared with the current technique.

FIG. 6 shows a multi-grating target 600, comprising individualsub-targets 632 to 635. As in the target of FIG. 4, the four sub-targetscomprise two overlay gratings for measurement in the X direction and twooverlay gratings for measurement in the Y direction. Instead ofproviding a fixed overlay bias within each grating, however, amulti-step or continuous variation of bias including negative values,positive values and intermediate values is provided. Gratings 632 and635 have bias values increasing with Y and X,X and Y, respectively.Conversely, gratings 633 and 634 have bias values decreasing with X andY, respectively. It is a matter of design choice, whether the target 600and the individual sub-targets have the same dimensions as a knowntarget, or are made larger or smaller.

FIG. 7 shows schematically the corresponding image 740 captured onsensor 23 in the apparatus of FIG. 3. Reference signs 742 to 745indicate the intensity image regions corresponding to the individualsub-targets. Due to the variation of bias over each sub-target, theintensity varies, rather than being constant within each region. Insteadof regions of interest ROI, one can imagine “lines of interest” LOI,aligned with the direction of variation of bias, as shown. The manner ofprocessing the intensity information to obtain overlay measurements willbe described later. First, various possible implementations of thecontinuous-bias targets will be illustrated.

In FIG. 8 (a), using the sub-target 632 of FIG. 6 as an example, thesub-target comprises an overlay grating having features 802 printed overunderlying features 804. In all of the examples herein, it will beunderstood that features are shown enlarged for illustrative purposesonly. The real gratings may have tends of hundreds of lines. Thefeatures 804 and 802 are formed so as to lie not parallel, but with asmall angle of deviation between them, for example between 0.1 and 0.5°,for example 0.35°. In the example, the underlying features 804 have beenprinted at an angle to the y-axis. In a practical implementation, eitheror both layers may be rotatable relative to the axis. As a result,within orientation of lines as shown, a bias value d varies from zeroacross the middle of the target to positive values above the middle andnegative values below the middle. The bias value is in the X direction,and varies continuously in the Y direction across the target In analternative implementation, rather than sloping lines, one or both linesmay be staggered in a series of fine steps in bias may be implemented.

In the situation shown in FIG. 8(a), the overlay error OV is zero, sothat asymmetry A is zero along the same line where bias is zero. On theother hand, referring to FIG. 8(b), when overlay in the X direction isnot zero, the line where asymmetry is zero is shifted in the Ydirection.

FIG. 9 shows the four sub-targets 632 to 635, having the form shown inFIG. 8, and with the appropriate orientation and polarity of biasvariation.

FIG. 10 shows another example target design having continuous ormulti-step bias in four sub-targets, numbered 1032 to 1035. Eachsub-target in this case has the form of a rectangle, rather than asquare. Otherwise, the orientation of the grating of sub-targets 1032 to1035 are the same as in the targets 632 to 635, respectively, in theexample of FIGS. 6 to 9. FIG. 11 shows the corresponding image, withregions 1142 to 1145 corresponding to the sub-targets 1032 to 1035.Again, the variation in bias causes a variation of intensity over theimage of each sub-target, rather than a single region of interest havinguniform intensity.

FIG. 12 (a) illustrates one possible implementation of the rectangularcontinuous bias sub-targets of FIG. 10. A grating of only a few lineshaving exaggerated bias variation is shown, for the purpose ofexplanation. Line features 1202 in a top layer of the sub-target, areprinted overlying features 1204 in a bottom layer. Instead of rotatingone or both sets of line features to obtain a bias variation, in thisexample, the pitch (period) of the grating in the top and bottom linesis different. Features 1202 in the top layer are arranged with a pitchP1, while features 1204 in the bottom they are arranged with a slightlysmaller pitch P2. This causes a linear variation of bias d as shown bythe line 1206 and the graph of FIG. 12(b). The line has a slope S. Incontrast to the rotation example of FIGS. 6 to 7, the variation of bias,and hence the lines of interest LOI, is parallel to the direction ofperiodicity of the grating.

Also shown in the graph is a sinusoidal curve representing the variationof asymmetry A across the grating. Assuming overlay error to be zero,the bias d and asymmetry A our zero along the same line, as indicated.In the presence of overlay error, this relationship breaks down. Inorder to be able to determine overlay error, a shift of the zeroasymmetry point relative to the known line of zero bias can be measured.To do this from a single target, however, would require very precisemeasurement of the position of the target, to know the position of thezero bias line. As will be illustrated with FIG. 13, the provision ofcomplementary pairs of sub-targets with opposite bias variation allowsmore accurate measurement of overlay, and also makes the measurementrobust against variations attributable to process effects, and asymmetryin the measurement apparatus.

FIG. 13 shows the sub-targets 1032 and 1034, under conditions (a) ofzero overlay error and (b) of non-zero overlay error in the X direction.It will be understood that the same explanation will apply to theY-direction sub-targets 1033 and 1035. As labeled, sub-target 1032 hasthe larger pitch P1 over the small pitch P2. Bias d increasesprogressively with increasing X. By contrast, sub-target 134 has thesmaller pitch P2 over the larger pitch P1. Consequently, bias ddecreases progressively with increasing X.

Accordingly, when an overlay error is introduced, as shown at FIG. 13(b), the lines of zero asymmetry in the two sub-targets move equally butin opposite directions relative to the line of zero bias. By comparingthe asymmetries measured from the intensity images 1142 and 1144 of thepair of sub-targets, overlay error can be measured.

The complete overlay measurement method will be described now withreference to FIG. 14 using the example of the target 1000 of FIGS. 10 to13. The method may be applied to the continuous bias targets of FIGS. 6to 9, and to alternative examples including ones illustrated anddescribed further below. For the sake of example, FIG. 15 illustratesdetail of the method in (a) the measurement of asymmetry variation overthe length of the sub-target 1032 of target 1000 and (b) the measurementof asymmetry variation over the length of the sub-target 1034.

In FIG. 14 at step S1, the substrate, for example a semiconductor wafer,is processed through a lithographic apparatus, such as the lithographiccell of FIG. 2, one or more times, to create an overlay target includingthe sub-targets 1032-1035. At S2, using the metrology apparatus of FIG.3, an image of the sub-targets 1032 to 1035 is obtained using only oneof the first order diffracted beams (say −1). At step S3, a second imageof the overlay targets using the other first order diffracted beam (+1)is captured in a second image. While we refer for simplicity to a singleimage, multiple images may be taken, either under the same illuminationconditions or under different conditions, to increase the informationavailable, and achieve a desired level of measurement performance.Illumination conditions may vary for example in wavelength and/orpolarization.

Note that, by including only half of the first order diffractedradiation in each image, the ‘images’ referred to here are notconventional dark field microscopy images. The individual overlay targetlines of the overlay targets will not be resolved. Each overlay targetwill be represented simply by an area of a certain intensity level.

In step S4, intensity values are sampled along one or more lines ofinterest LOI illustrated in FIGS. 15 (a) and (b) aligned with thedirection of variation of bias within the image of each componentoverlay target.

In step S5 the variation of asymmetry over each sub-target is determinedthe processor PU by comparing the intensity values obtained for +1 and−1 orders for each sub-target 1032-1035. The by simple subtraction, orin ratio form, as is known. Techniques similar to those used in knownmethods can be applied for identifying the regions of interest andaligning the +1 and −1 images to pixel accuracy, can be applied.

It is a matter of implementation, whether intensity values for all linesof interest LOI are combined before being compared to derive asymmetry,or whether asymmetry values are derived along lines of interest, andthen combined to obtain an average asymmetry. As illustrated in FIGS. 15(a) and (b), the presence of a variation of bias of known slope in thetarget allows additional information, and data verification to beperformed as a preliminary step in step S5. For example, asymmetrysample values 1500 can be fitted to a predicted linear or sinusoidalrelationship (curve 1502, 1504). Edge regions of the target can beclearly identified by deviation from the fitted curve, as seen at 1506.Anomalous values 1508, 1510 can be identified similarly. The circledsample values can be excluded from calculation. Where the bias and/oroverlay error drive the signals into non-linear regions of the asymmetrycurve, a linear section of the response can be identified, and onlyvalues from this section used, if desired. Such filtering can beperformed in the asymmetry values, as shown, and/or in the intensityvalues, prior to calculating asymmetry.

As will be illustrated further below, targets of suitable design caninclude “anchor points”, so that this preprocessing can also improvealignment of features between the sub-target images.

In step S6 the measured intensity asymmetries for a number of overlaytargets are used, together with knowledge of the known variation ofoverlay biases of those overlay targets, to calculate one or moreperformance parameters of the lithographic process in the vicinity ofthe overlay target T. A performance parameter of great interest isoverlay.

The current overlay calculation method was described above, withreference to Equations 1, 2 and 3. Different methods can be appliedusing the continuous bias/multiple biased targets of the presentdisclosure.

FIG. 15 illustrates one method, based on fitting a curve with theexpected behavior. In the linear example illustrated:

A _(PB) =a _(PB) *X+b _(PB) ; A _(NB) =a _(NB) *X+b _(NB); or

A _(PB) =K*(OV+S*X)+b _(PB) ; A _(NB) =K*(OV−S*X)+b _(NB)

Where APB and ANB are the asymmetry values at each point X along thesub-target 1032 having positive bias variation and along the sub-target1034 having negative bias variation. Factors aPB, bPB, aNB, bNB dependupon the case. In the ideal case, it is expected that aPB=aNB. Thesecond equations translate these factor into terms of theprocess-dependent factor K mentioned already above, the unknown overlayerror OV, and the known slope S of the bias variation. It is assumedthat the slope S is the same between the two sub-targets, differing onlyin sign.

In case a sinusoidal model would be applied, the equations become:

A _(PB) =b _(PB) +K*sin(OV+S*X); A _(PB) =b _(NB) +K*sin(OV−S*X);

In FIG. 15 (b), the curve 1502 which is the asymmetry variation for thesub-target 1032 with positive bias variation is superimposed on the samegraph as the asymmetry variation for the sub-target 1034 with negativebias variation. Due to a positive overlay error OV, the line of zeroasymmetry has moved to the left of the line of zero bias for thesub-target 1032, and the line of zero asymmetry for the sub-target 1034has moved to the right. To determine overlay, the processor calculatesthe shift (xs) between the zero points of the asymmetries APB and ANB.Then it is a simple matter to calculate overlay from the shift xs andthe known slope of the target. In the case of a linear model, this is:

OV=xs/S

A sinusoidal model can be applied, if desired.

In an alternative implementation, overlay is calculated for each spatialposition along the lines of interest, for example as follows:

K=(A _(PB) −A _(NB))/S*X

OV=(A _(PB) +A _(NB))/[S*X*(A _(PB) −A _(PB))]

The results from all the positions can be combined into a single overlaymeasurement. Again, a sinusoidal model can be applied, if desired. Asmentioned above, it is a matter of implementation, whether such acalculation is performed separately for various lines of interest LOI,and then combined, or whether pixel values are averaged in the directiontransverse to the lines of interest, before being used in thecalculation. A filtering step to remove nonlinear regions(non-sinusoidal regions) and outliers can be applied in the overlaycurve, before the results are combined, based on the principlesillustrated for the asymmetry curves in FIG. 15.

Regions with equal bias should have the same asymmetry response on bothcurves, but deviations can be caused by misalignment and optical and/orprocessing effects. This will introduce inaccuracy in the methods asdescribed so far above. Accordingly, in some embodiments, features areincluded that may be used as “anchor points” to facilitate alignment ofthe asymmetry curves, before they are combined to calculate overlay.

In the example of FIGS. 16 and 17, anchor points are embedded within thevariation of bias across the target. Part of a modified multi-gratingtarget 1600 is shown, which is a modified version of the one shown inFIGS. 10 to 13. Two sub-targets 1632 and 1634 are shown. In a middlesection of each target, the top and bottom grating pitches P1 and P2 arethe same as in the target 1000. However, in other regions, the pitchesP1 and P2 are reversed, so that the slope of the bias variation changes(in this example, reverses) at known points in the structure. In theexample, the slope reverses at positions X1 and X2, as marked. In anexample, which is not shown in FIG. 16, the bottom grating of target1632 has the pitch P2, and the top grating of target 1632 comprises tworegions, both overlapping bottom grating with pitch P2, wherein thefirst region of the top grating of target 1632 has a pitch P1 smallerthan P2 of the bottom grating of target 1632, and the first region ofthe top grating of target 1632 has a pitch P3 larger than the pitch P2of the bottom grating of target 1632.

FIG. 17(a) shows the slope reversals when the bias d is plotted againstposition X for the sub-target 1632 (curve 1702, solid lines) sent forthe sub-target 1634 (curve 1704, broken lines). Note that the variationof bias is cyclic, for the purposes of asymmetry measurement, indicatedby the step 1706.

In FIG. 17(b) the sinusoidal variation of asymmetry is shown, asmeasured from the image 1142 of sub-target 1632 (curve 1712, solidlines) and from the image 1144 of sub-target 1634 (curve 1714, brokenlines). A difference in the position of zero crossings between thesecurves is caused by the shift xs corresponding to overlay error, but itmay also be caused by an error in the image alignment, or caused byprocessing effects or optical effects in the apparatus. Because theslope changes incorporated in the target 1600 provide recognizableanchor points deviating from the sinusoidal form of curves 1712 and1714, this source of error can be identified by misalignment between theanchor points, indicated for example at 1716 in FIG. 17(b).

In FIG. 17(c), the curves 1712 and 1714 have been re-plotted as curves1722 and 1724, shifted to align the anchor points so that these pointswith the known same bias and same X position are aligned. From thesecurves, the shift xs between the zero crossings of the curves iscalculated, and overlay is calculated as described above.

When multiple anchor points are provided, an average of their relativeshifts can be used to obtain the best fitting of the curves. The numberof anchor points may be less than two or more than two. In principle, agrating of the type shown having three or more changes of slope could beused by itself, without requiring a second grating for comparison. Thisis because sub-targets having the desired sequence of positive biasvariation and negative bias variation can be found within the sameextended structure. Accordingly, “sub-targets” should be interpreted toinclude overlapping regions within a single grating structure. While theabove example includes reversals of slope as anchor points, other typesof anchor point can be included, including small regions of constantbias. Regions of constant bias and reversals of slope could be includedin the same target, either at the same or at different locations. Notethat regions of constant bias are examples of changes of slope, andchanges of slope is not limited to reversals of slope. The slope changesmay be designed to occur in a region where the asymmetry is sensitive tobias change, as in the example shown. Sensitivity does depend on processeffects and optical effects, and therefore this cannot be perfectlycontrolled.

FIG. 18 shows an example of a substructure having not a continuous biasvariation, but a stepwise variation of bias. Provided the variation ofbias is known, the appropriate curve can be fitted to the observedintensities and asymmetries.

FIG. 19 shows a further variation in which each sub-target of amulti-grating target has regions of different bias. In this example, theX direction bias changes from −10 nm to +10 nm, in the Y direction.

FIG. 20 illustrates a target or sub-target 2000 in which L-shaped linefeatures are used to provide a grating with both X and Y diffraction. InFIG. 21, such a design is adapted to provide a target 2100 in whichstepwise variation of bias is provided along lines of interest indifferent segments of the target. Biases ranging from −10 nm to +10 nmin the Y direction are labeled. FIG. 22 shows a target 2200 in which theL-shaped line features of a bottom grating are slightly rotated, in themanner of FIG. 6, thereby providing continuous variation of bias acrossthe target. The targets 2120 and 2200 are provided with alignmentfeatures 2102 and 2202, respectively, to assist in alignment of theimages for extracting intensity measurements.

FIG. 23 illustrates a larger target 2300 incorporating the features oftarget 2200, but repeated around a square. FIG. 24 illustrates a target2400 in which four sub-targets having the form of target 2200 butrotated are provided.

The above are only some examples of target designs that can beimplemented applying the concepts disclosed herein. The methodsdescribed are only example methods of how signals from these targets canbe processed to obtain improved overlay measurement, and/or improvedutilization of space on a substrate.

While the targets described above are metrology targets specificallydesigned and formed for the purposes of measurement, in otherembodiments, properties may be measured on targets which are functionalparts of devices formed on the substrate. Many devices have regular,grating-like structures. The terms ‘target grating’ and ‘target’ A usedherein do not require that the structure has been provided specificallyfor the measurement being performed. Further, pitch P of the metrologytargets is close to the resolution limit of the optical system of thescatterometer, but may be much larger than the dimension of typicalproduct features made by lithographic process in the target portions C.In practice the lines and/or spaces of the overlay gratings within thetargets may be made to include smaller structures similar in dimensionto the product features.

In association with the physical grating structures of the targets Arealized on substrates and patterning devices, an embodiment may includea computer program containing one or more sequences of machine-readableinstructions describing methods of measuring targets on a substrateand/or analyzing measurements to obtain information about a lithographicprocess. This computer program may be executed for example within unitPU in the apparatus of FIG. 3 and/or the control unit LACU of FIG. 2.There may also be provided a data storage medium (e.g., semiconductormemory, magnetic or optical disk) having such a computer program storedtherein. Where an existing metrology apparatus, for example of the typeshown in FIG. 3, is already in production and/or in use, the inventioncan be implemented by the provision of updated computer program productsfor causing a processor to perform the steps S1-S6 and so calculateoverlay error.

The program may optionally be arranged to control the optical system,substrate support and the like to perform the steps S1-S6 formeasurement of asymmetry on a suitable plurality of targets.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), A well A particle beams, such A ion beams or electronbeams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of components, including refractive,reflective, magnetic, electromagnetic and electrostatic components.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description by example, and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

1. A target formed on a substrate, the target comprising: a first pairof sub-targets comprising: a first sub-target; and a second sub-target;and a second pair of sub-targets comprising: a third sub-target; and afourth sub-target, wherein the first and second sub-targets are arrangedadjacent to each other and the third and fourth sub-targets are arrangedon opposite sides of the first and second sub-targets, wherein eachsub-target comprises an overlay grating with a top layer and a bottomlayer with different pitches, and wherein the target comprises acontinuous variation of bias values across each sub-target.
 2. Thetarget of claim 1, wherein: features in a top layer of the firstsub-target are arranged with a first pitch; features in a bottom layerof the first sub-target are arranged with a second pitch; and the firstpitch is larger than the second pitch.
 3. The target of claim 1,wherein: features in a top layer of the second sub-target are arrangedwith a first pitch; features in a bottom layer of the second sub-targetare arranged with a second pitch; and the first pitch is smaller thanthe second pitch.
 4. The target of claim 1, wherein the continuousvariation of bias values across each sub-target is associated with anasymmetry-related property of the target.
 5. The target of claim 1,wherein the sub-targets in the first and second pairs have equal andopposite bias variations.
 6. The target of claim 1, wherein the firstand second sub-targets have a same orientation that is opposite to anorientation of the third and fourth sub-targets.
 7. The target of claim1, wherein the sub-targets of the first and second pairs arerectangular.
 8. A patterning device for use in a lithographic apparatus,the patterning device comprising: portions that define one or moredevice patterns; and portions that define one or more metrologypatterns, wherein the one or more metrology patterns comprise a target,the target comprising: a first pair of sub-targets comprising: a firstsub-target; and a second sub-target; and a second pair of sub-targetscomprising: a third sub-target; and a fourth sub-target, wherein thefirst and second sub-targets are arranged adjacent to each other and thethird and fourth sub-targets are arranged on opposite sides of the firstand second sub-targets, wherein each sub-target comprises an overlaygrating with a top layer and a bottom layer with different pitches, andwherein the target comprises a continuous variation of bias valuesacross each sub-target.
 9. The patterning device of claim 8, wherein:features in a top layer of the first sub-target are arranged with afirst pitch; features in a bottom layer of the first sub-target arearranged with a second pitch; and the first pitch is larger than thesecond pitch.
 10. The patterning device of claim 8, wherein: features ina top layer of the second sub-target are arranged with a first pitch;features in a bottom layer of the second sub-target are arranged with asecond pitch; and the first pitch is smaller than the second pitch. 11.The patterning device of claim 8, wherein the continuous variation ofbias values across each sub-target is associated with anasymmetry-related property of the target.
 12. The patterning device ofclaim 8, wherein the sub-targets in the first and second pairs haveequal and opposite bias variations.
 13. The patterning device of claim8, wherein the first and second sub-targets have a same orientation thatis opposite to an orientation of the third and fourth sub-targets. 14.The patterning device of claim 8, wherein the sub-targets of the firstand second pairs are rectangular.
 15. A metrology apparatus comprising:an illumination system configured to illuminate with radiation a targetformed on a substrate; and a detection system configured to detectscattered radiation arising from illumination of the target, the targetcomprising: a first pair of sub-targets comprising: a first sub-target;and a second sub-target; and a second pair of sub-targets comprising: athird sub-target; and a fourth sub-target, wherein the first and secondsub-targets are arranged adjacent to each other and the third and fourthsub-targets are arranged on opposite sides of the first and secondsub-targets, wherein each sub-target comprises an overlay grating with atop layer and a bottom layer with different pitches, and wherein thetarget comprises a continuous variation of bias values across eachsub-target, and wherein the metrology apparatus is configured to: obtaina plurality of asymmetry measurements from locations on the target; fitthe plurality of asymmetry measurements to at least one expectedrelationship between asymmetry and a performance parameter, based on abias variation between the locations on the target; and derive a measureof the performance parameter from the fitting.
 16. The metrologyapparatus of claim 15, wherein: features in a top layer of the firstsub-target are arranged with a first pitch; features in a bottom layerof the first sub-target are arranged with a second pitch; and the firstpitch is larger than the second pitch.
 17. The metrology apparatus ofclaim 15, wherein: features in a top layer of the second sub-target arearranged with a first pitch; features in a bottom layer of the secondsub-target are arranged with a second pitch; and the first pitch issmaller than the second pitch.
 18. The metrology apparatus of claim 15,wherein the continuous variation of bias values across each sub-targetis associated with an asymmetry-related property of the target.
 19. Themetrology apparatus of claim 15, wherein the sub-targets in the firstand second pairs have equal and opposite bias variations.
 20. Themetrology apparatus of claim 15, wherein the first and secondsub-targets have a same orientation that is opposite to an orientationof the third and fourth sub-targets.